Hierarchical conductive metal-organic framework films enabling efficient interfacial mass transfer

Heterogeneous reactions associated with porous solid films are ubiquitous and play an important role in both nature and industrial processes. However, due to the no-slip boundary condition in pressure-driven flows, the interfacial mass transfer between the porous solid surface and the environment is largely limited to slow molecular diffusion, which severely hinders the enhancement of heterogeneous reaction kinetics. Herein, we report a hierarchical-structure-accelerated interfacial dynamic strategy to improve interfacial gas transfer on hierarchical conductive metal-organic framework (c-MOF) films. Hierarchical c-MOF films are synthesized via the in-situ transformation of insulating MOF film precursors using π-conjugated ligands and comprise both a nanoporous shell and hollow inner voids. The introduction of hollow structures in the c-MOF films enables an increase of gas permeability, thus enhancing the motion velocity of gas molecules toward the c-MOF film surface, which is more than 8.0-fold higher than that of bulk-type film. The c-MOF film-based chemiresistive sensor exhibits a faster response towards ammonia than other reported chemiresistive ammonia sensors at room temperature and a response speed 10 times faster than that of the bulk-type film.

In this manuscript, the authors report the fabrication of hollow structured c-MOFs for enhanced interfacial mass transfer and their applications as gas sensors with fast responses. Their synthetic strategy for fabricating hollow c-MOFs is very interesting and useful. However, further explanation is required to support the authors' claims on fast mass transfer in hollow c-MOFs. Also, the delivery of the manuscript is insufficient due to several omissions and errors in the description. In this regard, the following revisions are required for this work to meet the standard of publication on Nat. Commun.
1. The authors explain that interfacial mass transport is faster in hollow structures, assuming that parallel flow is supplied along the surface. However, in gas sensors, analytes can be transported to the surface from all directions, including vertical direction. Is the authors' claim valid under these circumstances? How significant is parallel mass transport along the surface for the speed of real gas-solid reactions?
2. The authors discussed the relationship between enhanced mass transport and porous structure,                  internal pores in MOFs? As the crystalline porous structure is a unique feature of MOFs, consideration of contribution from micropores of MOFs, together with macroscopic hollow structure, would provide a deeper understanding of the MOF-based porous medium.
3. More details on the sensing measurements setup should be provided. How was the gas concentration regulated from the dropped ammonia water? How was the gas supplied to the sensor, and at what flow rate?
               5. While I agree with the faster response speed of hollow samples, the authors should mention that the calculated response time values can be inaccurate in several samples. As some samples did not reach saturation during exposure, the calculated response time will differ depending on gas exposure duration.
6. The authors should provide explanation of Fig. 3g-i in the main text.
Reviewer #2 (Remarks to the Author): In this manuscript, the authors reported a hierarchical-structure-accelerated interfacial dynamic (HSAID) strategy to efficiently improve interfacial gas transfer on conductive metal-organic framework (c-MOF) films via the construction of hierarchical porous structures. In addition, the Zn-HHTP-H film was integrated into a chemiresistive ammonia sensor and showed good performance. These innovative results are logically presented and would raise wide interest. This work is suitable for publication in Nature communication after addressing the following issues. 1.The authors only supplied the photo of ZIF-67 film coated on Si/SiO wafer (Fig. S5). To make direct comparisons, the photographs of Zn-HHTP-HS and Zn-HHTP-B films on Si/SiO2 wafer should be supplied in the manuscript. 2.There are still some spelling and grammar mistakes in this manuscript. Please read carefully and correct all of them, such as on page S5, the unit of the aging time was missing, which would confused other researchers repeating this experiment. 3.On page 6, Fig. 2c-g don't match the description 'All these hierarchical c-MOF films exhibited a highly crystalline structure and intrinsic electrical conductivity, while the inside was hollow', please check carefully and make a correction. 4.On page 7, Supplementary Fig. S9b, 9d, 9e cannot match the contents in support information, please check carefully and make a correction. 5.On page S20, the authors claim that HHTP could not be protonated, when only organic solvents existed such as EtOH or DMF. This statement should be made a reconsideration. The method of sacrificial ZIF-template for MOF film conversion is the key part of this article, please discuss in detail about the water and solvent factors of this process. General comment: In this manuscript, the authors report the fabrication of hollow structured c-MOFs for enhanced interfacial mass transfer and their applications as gas sensors with fast responses. Their synthetic strategy for fabricating hollow c-MOFs is very interesting and useful. However, further explanation is required to support the authors' claims on fast mass transfer in hollow c-MOFs. Also, the delivery of the manuscript is insufficient due to several omissions and errors in the description. In this regard, the following revisions are required for this work to meet the standard of publication on Nat. Commun.
Reply: We sincerely appreciate this reviewer for the encouraging comments and the positive recommendation for publication after revision. According to your valuable suggestions, we here further addressed all the issues carefully that make our work of great interest to the broad readership of Nat. Commun.
Comment 1: The authors explain that interfacial mass transport is faster in hollow structures, assuming that parallel flow is supplied along the surface. However, in gas sensors, analytes can be transported to the surface from all directions, including vertical direction. Is the authors' claim valid under these circumstances? How significant is parallel mass transport along the surface for the speed of real gas-solid reactions?
Reply: We fully understand the reviewer's concerns about mass transport in the vertical direction. As shown in our schematic of interfacial mass transport on a solid porous film ( Fig. 1a and 1b), The concentration gradient-induced molecular diffusion (vertical direction) and the mass transport induced by surface convection (horizontal direction) have both already been taken into account in our study. Herein, based on the systematic numerical simulations and experimental results, it can be concluded that the interfacial mass transfer mainly depends on the slow molecular diffusion for the bulk-type Zn-HHTP-B film. However, for the hierarchical Zn-HHTP-H film, the interfacial mass transfer depends on both the molecular diffusion and surface convection induced mass transport. Compared with the Zn-HHTP-B film, the enhanced interfacial mass transport on hierarchical hollow Zn-HHTP-H films should be attributed to hierarchical hollow structure induced surface convection (i.e., parallel mass transport along the surface). First, For the interfacial mass transport, two factors were involved (one is the velocity vector of the gas fluid (left in Fig.1), ne is the concentration field of gas molecules (right in Fig.1)). For the concentration field of gas molecules, the gas molecules are indeed transported from the bulk gas environment to the film surface from vertical direction, as mentioned by the reviewer. However, for the velocity vector of the gas fluid, it consists of the concentration gradient-induced molecular diffusion (vertical direction) and the surface convection (V in horizontal direction). However, previous studies mainly focused on the concentration gradient-induced molecular diffusion in the vertical direction ( , which has led to parallel mass transport induced by surface convection being overlooked. Herein, based on our study, the mass transport induced by surface convection was found to be able to greatly enhance the interfacial mass transport. Second, for the surface fluid behaviour at the boundary layer between the bulk gas environment and porous c-MOFs medium, the surface velocity U was described by the famous Beavers and Joseph theory (J. Fluid Mech., 1967, 30 197-207) as follows: As shown in the computational fluid dynamics simulation, the parallel surface velocity Us at the film surface is 3.94×10 -5 m s -1 for the Zn-HHTP-H film, which is over one order of magnitude higher than vertical velocities (~10 -6 m s -1 ), indicating the Zn-HHTP-H film allows higher mass transfer via parallel convection rather than diffusion. Moreover, the Péclet number characterizing the ratio of the parallel advection to the diffusion (Trautz, M., et al. Ann. Phys.1935, 414, 333-352), i.e., UL/D, is at least one order of magnitude higher for the Zn-HHTP-H film (0.2) than for the Zn-HHTP-B film (0.027). This means that the diffusion dominates the mass transport for the Zn-HHTP-B films. Therefore, we can focus on the enhanced mass transport for the Zn-HHTP-H film by analyzing the parallel convection velocity.
Third, all the Zn-HHTP films have the same crystalline structures and chemical composition, as demonstrated by the XRD patterns, the FT-IR spectra, XPS, and TGA curves ( Fig. S22-S25). The three Zn-HHTP films possess different sizes of hollow cavities (Fig. S13, S15, S16 and S20), which lead to much different gas permeability ( Fig. 3f-3i). Although all the Zn-HHTP samples possessed similar film thicknesses of ~ 500 nm, the hollow Zn-HHTP-H (9.1 s) film-based sensor displayed a much faster response speed than those of Zn-HHTP-HS (41.9 s) and Zn-HHTP-B (99.3 s).
Obviously, for the Zn-HHTP-B film, the slow molecular diffusion on the surface results in slow mass transfer (slow response speed). After introducing hollow cavities inside the film (Zn-HHTP-H film), the gas permeability of the film was greatly enhanced and extra nonzero convection velocities were generated on the surface. The increased convection on the hollow film surface in turn enlarged the concentration gradient and enhanced the transport from the environment to the Zn-HHTP-H film surface.
In conclusion, the faster sensing response in hierarchical c-MOF film than that in the bulk-type film should be attributed to the hollow structure-induced high permeability greatly promotes interfacial mass transfer. We believe that the present results can establish the relationship between hollow induced permeability in porous c-MOF and its interfacial mass transfer performance.
Comment 2: The authors discussed the relationship between enhanced mass transport              effect of the size or volume of internal pores in MOFs? As the crystalline porous structure is a unique feature of MOFs, consideration of contribution from micropores of MOFs, together with macroscopic hollow structure, would provide a deeper understanding of the MOF-based porous medium.
Reply: We appreciate the reviewer for keenly pinpointing the major contribution of this work and appreciate this constructive comment. In fact, the effect of the intrinsic micropores on MOFs has already been considered. This research is aimed at establishing a relationship between the hierarchical structure in porous c-MOFs film and interfacial mass transfer. For such a purpose, the porous nature of the c-MOFs nanofilm itself is the premise, otherwise the macroscopic hollow structure inside the film cannot interact with the external environment.
First, from an experimental point of view, all the Zn-HHTP films have the same crystalline structures and chemical composition, as demonstrated by the XRD patterns (Fig. S22), the FT-IR spectra (Fig. S23), XPS (Fig. S24), and the TGA curves (Fig.

       
that all the Zn-HHTP films possessed the same 1.1 nm micropores (Fig. S26). The only significant difference in the Zn-HHTP films is their macroscopic hollow structure (Fig.  S15, S16 and S20), which results in various gas permeability of films and significantly different mass transfer speeds in gas sensing (Fig.3f-3i and Fig.5c). All the results demonstrated that the enhanced mass transport in hierarchical c-MOF films mainly due to the effect of macroscopic hollow structure in the films. The relevant statements were shown on page 7-8 in the revised manuscript and the relevant characterizations are shown on page S31-S35 in the revised Supporting Information.
Second,           point of view, in our model, the internal pores of the c-MOFs framework have already been taken into account and were modelled as the porous medium model with a fixed permeability (2×10 15 m 2 ) and a porosity (0.2 0.3, see Supplementary Table S3), while the flow in the hollow is solved directly. For example, the bulk-type Zn-HHTP-B film was modelled as a porous medium with a porosity ( a = 0.2) and a BET (165.3 m 2 g -1 ), which strictly follows the data obtained from the BET gas adsorption measurement. Moreover, Khollow from our modified permeability model consists of the contribution of intrinsic micropores, i.e., Kbulk, and hollow structure, i.e., Comment 3: More details on the sensing measurements setup should be provided. How was the gas concentration regulated from the dropped ammonia water? How was the gas supplied to the sensor, and at what flow rate?
Reply: Thanks for the constructive comments, and we added detailed methods for gas sensing experiments to the revised manuscript and supporting information.
First, the volume of ammonia water for gas concentration regulation was described as follows: V is the volume of the gas sensing test chamber, which is 20 L; C is the gas concentration, ppm; M is liquid molecular weight; D is the liquid density, g cm -3 ; P is the liquid purity; TR is the room temperature, °C; TB is the temperature inside the gas sensor test chamber, °C; Vx is the volume of fluid to be injected. Taking 50 ppm ammonia gas as an example, Vx is calculated to be 3.2 uL according to the above formula.
Second, the detailed method for the gas supplied to the sensor is following: Using a micro-syringe to take a certain volume of ammonia water and put it into a hightemperature evaporating dish (150 °C) inside the gas-sensing test chamber. Then ammonia water will evaporate into ammonia gas. Under the continuous agitation of the fan, the ammonia gas quickly would fill the entire test chamber and diffuse to the surface of the sensor. The overall gas flow rate defaults to 20 L min -1 .
Third, the diagram of the experimental setup is shown as follows: Supplementary Fig. S33. (a) Diagram of the gas-sensing test device and (b) schematic diagram of airflow diffusion.
The relevant statements were shown in page 18 in revised manuscript and page S7 and S42 in revised Supporting Information.
Comment 4:               Reply: Thanks for the timely comments, and we modified the calculation formula in table S1. The relevant statements were shown in page S61 in Supporting Information.
Comment 5: While I agree with the faster response speed of hollow samples, the authors should mention that the calculated response time values can be inaccurate in several samples. As some samples did not reach saturation during exposure, the calculated response time will differ depending on gas exposure duration.
Reply: Thank you for the reviewer's support of the main point and constructive comments. For the Zn-HHTP sensors in Fig. 5c, the response of all the three sensors towards ammonia gas has reached saturation, and the calculation of the response time is based on the time to reach 90% of the response value. But, as the reviewers pointed out, some sensors indeed did not reach saturation in response to ammonia (Fig. 6e and 6f). In Fig.  6e, the response of the hollow Pc-CuZn-H to ammonia has reached saturation, and the calculated response time was 9.8 s, while the response time of bulk Pc-CuZn-B was longer than 53.1s. The situation in Fig. 6f is similar. The response of the hollow Co-HHTP-H to ammonia has reached saturation, and the calculated response time was 19.2 s, while the response time of the bulk Co-HHTP-B was longer than 75.1s. All the above results demonstrate that the hierarchical hollow nanostructures would promote interfacial mass transfer, leading to a fast response speed in gas sensing. Following the suggestion from the reviewer, we have modified Fig. 6e and 6f and the relevant statements in the revised manuscript as following: Upon exposure to 100 ppm ammonia, the PcCu-Zn-H film-based sensor displayed a very fast response time of 9.8 s, while the corresponding response time of PcCu-Zn-B (over 53.1 s) was significantly longer (Fig. 6e and Supplementary Fig. S50). Similarly, upon exposure to 20 ppm ammonia, the response speed of the Co-HHTP-H film-based sensor (19.2 s) was much higher than that of the Co-HHTP-B film (over 75.1 s) ( Fig.  6f and Supplementary Fig. S51). The relevant statements were shown on page 14 of the revised manuscript.
Comment 6: The authors should provide explanation of Fig. 3g-3i in the main text.
Reply: Following the suggestion from the reviewer, and the detailed explanation of Fig.  3g-i has been added in the revised manuscript. Next, the theoretical pressure-driven flow was analysed to understand the film permeability via commercial CFD software FLUENT (Fig. 3g-3iSupplementary Figs. S29-S30). The simulated N2 fluxes of the Zn-HHTP-H and Zn-HHTP-HS films were 10.2 and 2.2 times higher than that of the Zn-HHTP-B film, respectively, consistent with the experimental permeability results (Fig. 3g-3i). The relevant statements were shown on page 8 of the revised manuscript.
General comment: In this manuscript, the authors reported a hierarchical-structureaccelerated interfacial dynamic (HSAID) strategy to efficiently improve interfacial gas transfer on conductive metal-organic framework (c-MOF) films via the construction of hierarchical porous structures. In addition, the Zn-HHTP-H film was integrated into a chemiresistive ammonia sensor and showed good performance. These innovative results are logically presented and would raise wide interest. This work is suitable for publication in Nature communication after addressing the following issues.
Reply: We sincerely appreciate this reviewer for his/her positive comments, recommendations, and detailed suggestions regarding the improvement of our manuscript. Moreover, we here further addressed the issues carefully that make our work of great interest to the broad readership of Nat. Commun.
Comment 1: The authors only supplied the photo of ZIF-67 film coated on Si/SiO wafer (Fig. S5). To make direct comparisons, the photographs of Zn-HHTP-HS and Zn-HHTP-B films on Si/SiO2 wafer should be supplied in the manuscript.
Reply: Following the suggestion from the reviewer, we added the photographs of assynthesized Zn-HHTP-HS and Zn-HHTP-B films on Si/SiO2 wafer in Figure S2. Moreover, we added the photographs of as-synthesized PcCu-Zn-B and Co-HHTP-B films on Si/SiO2 wafer in Figure S5. The relevant figures are shown in page S10 and S13 in Supporting Information.
Comment 2: There are still some spelling and grammar mistakes in this manuscript. Please read carefully and correct all of them, such as on page S5, the unit of the aging time was missing, which would confuse other researchers repeating this experiment.
Reply: Agreed, we have checked the manuscript and Supporting Information carefully and modified some mistakes. For the Supporting Information, we have added the unit of the time on page S5, and modified the captions and many relevant statements. For the manuscript, it has been polished by the native English speaker from Nature Publishing Group Language Editing. The relevant modifications are shown in the revised manuscript and Supporting Information.
Comment 3: On page 6, Fig. 2c-g don't match the description 'All these hierarchical c-MOF films exhibited a highly crystalline structure and intrinsic electrical conductivity, while the inside was hollow', please check carefully and make a correction.
Reply: We are sorry that we did not provide a correct description of Fig. 2c-g in our previous version. Herein we modify the relevant statements: Following the same sacrificial template synthetic method, the PcCu-Zn and Co-HHTP films with hierarchical nanostructures were synthesized (named PcCu-Zn-H and Co-HHTP-H, respectively, Fig. 2c-2g, Supplementary Fig. S5-S8). All these hierarchical c-MOF films exhibited intrinsic electrical conductivity, while the inside was hollow (Fig. 2f-2g, Supplementary Table S2). The relevant statements are shown on pages 5-6 in the revised manuscript.
Comment 4: On page 7, Supplementary Fig. S9b, 9d, 9e cannot match the contents in support information, please check carefully and make a correction. Reply: Agreed, and we modified the figure numbers. The Zn-HHTP-H film with ~500 nm cavities was obtained only at low water fractions               When the water fraction and temperature were increased (e.g. 37.5% and 40 °C, respectively), a Zn-HHTP film was formed with small cavities of ~175 nm (denoted as Zn-HHTP-HS film, Supplementary Fig. S13b, S13e and S16-S17). The relevant statements are shown on page 7 in the revised manuscript.
Comment 5: On page S20, the authors claim that HHTP could not be protonated, when only organic solvents existed such as EtOH or DMF. This statement should be made a reconsideration. The method of sacrificial ZIF-template for MOF film conversion is the key part of this article, please discuss in detail about the water and solvent factors of this process.
Reply: we feel sorry about the wrong description on page S20 in our previous version. herein we modify the relevant statements: First, when only organic solvent was used, no Zn-HHTP product was observed. It was supposed that the generated protons in organic solvent were too less to break the coordination bonds between Zn 2+ and MeIM linkers in the ZIF-8 crystals. Consequently, the etching step in equation 2 (3Zn(MeIM)2 + 6H +  6HMeIM + 3Zn 2+ ) was inhibited and finally no Zn-HHTP product was produced. The relevant statements are shown on page S20 in the revised Supporting Information.